1. Field of the Invention
This invention relates in general to a developing device used in an electrophotographic type image forming device. More particularly, the invention relates to a developing device of two-component developing manner where toner and carrier are stirred and a magnetic brush is formed on a sleeve in which magnets are enclosed to perform the development.
2. Description of Related Art
In a two-component developing device where the toner and the carrier are stirred and the magnetic brush of the carrier is made opposite to the surface of a photoreceptor to perform the development, a plurality of sleeves having magnets therein are provided, so that the development is performed while the magnetic brush crosses among the sleeves.
Because the development performance using only one single sleeve is better than that using a plurality of sleeves, a large linear velocity ratio of the sleeve to the photoreceptor is not required when the image forming device uses the aforementioned developing device. Therefore, even though for an imaging device where the photoreceptor rotates with a high speed to create a lot of sheets of output images per minute, no over load imparts on the developing motor or the bearings, etc., and therefore, the mechanical life time and the reliability can be improved. In addition, on the image, if the linear velocity ratio of the sleeve to the photoreceptor is small, the scavenging effect at the magnetic brush is strong enough that the effect of preventing the abnormal image, for example, the blur at the rear end of a solid image or the thinning of a line image, can be improved.
However, as the linear velocity ratio of the sleeve to the photoreceptor gets smaller and smaller, the scavenging effect with respect to the base surface of the photoreceptor gets weak and there is a problem that the base contamination is greatly created. Furthermore, because the developing performance is good, as the toner adhesion amount on the dot portion and the line portion increases, the unevenness of the adhesion amount also increases, so that the size of dot on the paper after being fixed becomes uneven. Additionally, even though on the vertical line portion, the scavenging effect gets weak and there is a problem that the roughness gets large easily after the fixing process.
Regarding the toner adhesion unevenness as described above, in general, an AC bias is applied to uniformize the toner adhesion, so as to obtain a smooth half tone image whose grain characteristic is suppressed. When a plurality of sleeves is used, the development performance gets better, but the development performance can be further increased if an AC bias is further applied. In this way, because the toner adhesion amount to the dot or the line is too much, the effect that the grain characteristic is suppressed by the AC bias cannot be obtained.
The Japanese Laid Open No. 2000-81790 provides a technology that the magnetization strength of the carrier of the two-component developer is regulated, so that a good image without being disturbed on the half tone portion can be obtained. In addition, according to the Japanese Laid Open No. 2000-293023, a blank pulse bias is used in the development using the two-component developer. Because the grain size of the consumed toner is different with time, a technology to set a mode such that the toner with a small grain size is forced to be consumed is provided. Furthermore, in the Japanese Laid Open No. 2000-321852, a photoreceptor with a surface layer having a volume resistance rate of 109˜1014 Ωcm is used. By using a developing bias that an AC voltage is overlapped to a DC voltage and the AC frequency is set above 4 kHz when developing, the charge injection from the carrier to the surface of the photoreceptor can be avoided.